The invention relates generally to the area of photovoltaic (PV) cells. More specifically, the invention relates to methods of making PV cells having a photo-active layer that includes cadmium telluride (CdTe). The invention also relates to methods of p-type doping of CdTe.
The solar spectrum “sunlight” contains a distribution of intensity as a function of frequency. It can be shown that the conversion efficiency for utilizing sunlight to obtain electricity via semiconductors is optimized for semiconducting band-gaps in the range vicinity between about 1.4 to about 1.5 electron volt (eV). The semiconducting band-gap of CdTe, is a good match for this requirement.
p-type CdTe is currently one of the common commercially used materials used in PV cells wherein the photo-active material is CdTe. Quite generally, in the interest of brevity of the discussions herein, PV cells including p-type CdTe as the photo-active material may be referred to as “CdTe PV cells.” Similarly, PV installations including CdTe PV cells would be referred to as “CdTe PV installations.” Commercial feasibility of large-scale PV installations including p-type CdTe PV cells has been demonstrated, and the cost of electricity obtained from such large-scale p-type CdTe PV installations is approaching grid parity. Commercial feasibility of smaller scale, that is, area confined, installations remains a challenge within the art due to the relatively poor overall efficiency of such smaller scale installations. Despite significant academic and industrial research and development effort, the best reported conversion efficiency “eB” of p-type CdTe PV cells has been stagnant at about 16.5% for close to a decade. This best reported conversion efficiency may be compared to the entitlement-efficiency of CdTe PV cells for the solar energy spectrum, which entitlement-efficiency is about 23%. The conversion efficiency numbers may further be compared to the “module” efficiency of typical currently available commercial large-scale p-type CdTe PV installations, which module efficiency is lower, and is about 11%.
Evidently, any improvement in p-type CdTe PV cell efficiency will result in an improvement in overall efficiency of corresponding CdTe PV installations. Such improvement will enhance the competitiveness of the CdTe PV installations compared to traditional methods of generating electricity, such as from natural gas or coal. It is evident that improvement in overall efficiency will enable p-type CdTe PV cell technology to successfully penetrate markets where small-scale area confined installations are required, such as markets for domestic PV installations.
Currently known methods for manufacturing p-type CdTe PV cells on commercially viable soda-lime glass substrates necessarily require relatively low temperature processes (performed typically at temperatures ˜<600 degrees Celsius (° C.)). Such necessity for low temperature processes, is one of the reasons why it has not been possible to enhance p-type doping levels beyond a doping level “cM,” wherein cM ˜5×1014 per cubic centimeter (/cm3). The inability to achieve p-type doping levels within CdTe, that are substantially in excess of cM, is among the factors resulting in the current stagnation of the conversion efficiencies at about eB.
There is a need within the art for methods of fabrication via which improved p-type CdTe PV cells having conversion efficiencies in excess of eM can be obtained. For any such methods to be commercially feasibly, they should be compatible with existing p-type CdTe PV cell fabrication process requirements such as for instance, the necessary requirement that the fabrication be performed at the relatively low temperatures mentioned above.
Methods whereby enhanced p-type doping levels within the photo-active CdTe material can be obtained, which methods are yet compatible with extant p-type CdTe PV cell fabrication processes, would therefore be highly desirable.